This invention relates to a photoelectric conversion element or image sensor used for an original document reading apparatus such as a facsimile system. In particular, it relates to a polychromatic image sensor having color separation and reading capability for two or more colors.
FIG. 1 illustrates a conventional polychromatic image sensor of the type as mentioned above. In FIG. 1, reference numeral 1 denotes a film of silicon oxide (SiO.sub.2), 2 denotes a P-type diffusion region (hereinafter referred to as a P-region), 3 denotes an N-type diffusion region (hereinafter referred to as an N-region), and 4 denotes a second P-region. Reference numerals 5, 6 and 7 respectively denote first, second and third electrode lead conductors respectively led out from the P-region 2, N-region 3, and P-region 4. Reference numerals 8 and 9 denote a first and a second PN-junction, respectively.
FIG. 2 is an equivalent circuit diagram of a polychromatic image sensor of the type as described above. A photodiode 10 is defined by the P-region 2 and the N-region 3 of FIG. 1. A photodiode 11 is defined by the N-region 3 and the P-region 4 of the sensor. The second electrode lead conductor 6 is coupled from the junction between the respective cathodes of the photodiodes 10 and 11. The first and third electrode lead conductors 5 and 7 are lead out from the respective anodes of the photodiodes 10 and 11.
The polychromatic image sensor of FIG. 1 may have the color separation and reading capability because, when light of various frequencies impinges the silicon, the coefficient of absorption varies as shown in FIG. 3. Consequently, there is a known relationship between wavelength of light and absorption into the silicon.
FIG. 4 shows in graph form a characteristic with the distance (x) from the light incident surface of silicon read along the x-axis and the light quantum reaching percentage (%) read along the y-axis with the number of light quantum .phi..sub.0, and wavelength as parameters. From FIGS. 3 and 4, it will be appreciated that as the wavelength of light decreases, the silicon absorbs light more effectively and, on the contrary, as the wavelength becomes longer, the light reaching distance in the silicon increases.
Accordingly, it will be apparent from the configuration of the polychromatic image sensor that the first PN-junction 8 can be reached by light of short wavelengths as well as light of long wavelengths, while the second PN-junction 9 cannot generally be reached by short wavelength light which has been already absorbed. Accordingly, information including short wavelength light can be obtained from the first photodiode 10, while information excluding short wavelength light can be obtained from the second photodiode 11.
The characteristics as shown in FIG. 5 have been reported as obtained in the polychromatic image sensor shown in FIG. 1. In FIG. 5, curves 10a and 11a denote the characteristics of the first and second photodiodes 10 and 11, respectively. It will be seen from the characteristics that the first photodiode 10 has high sensitivity to short wavelength light and, on the contrary, the second photodiode 11 has high sensitivity to long wavelength light.
In the polychromatic image sensor shown in FIG. 1, however, there is a drawback in that the first and second PN-junctions 8 and 9 are formed in parallel with the silicon oxide film 1. Specifically, to obtain a polychromatic image sensor having a desired characteristic, it is necessary to exactly form the depth or distance from the silicon oxide film to each of the first and second PN-junction planes and therefore, it is difficult to produce such a polychromatic image sensor having a desired characteristic. Moreover, it is difficult to accurately produce two or more PN-junctions.